A CCL-use copper foil must be improved in adhesion strength when bonding the copper foil to an insulating resin and satisfy electrical characteristics required as a printed circuit board, etching properties, heat resistance, and chemical resistance. For this reason, various countermeasures have been applied, for example, the bonding surface of the copper foil after foil formation (hereinafter sometimes also referred to as “untreated copper foil”) to be bonded to the insulating resin is roughened, the surface subjected to the roughening is further zinc (Zn) plated, nickel (Ni) plated, or the like, and further the surface subjected to the Zn plating or Ni plating or the like is chromate treated etc.
In IC (integrated circuit) mounting boards which drive liquid crystal displays used as display parts of personal computers, mobile phones, and PDAs (personal data assistants), recently there has been progress in increasing density. Accurate correct circuit configurations and heat stability at high temperature treatment are demanded in the manufacturing process of these.
In order to meet these demands, there is studied CCL in which an electrolytic copper foil for forming accurate conductive circuits and an insulating resin useable at a high temperature are adhered. One of the issues here is the improvement of the adhesion strength between the copper foil and the insulating resin at a high temperature for heat bonding of the copper foil and insulating resin at a high temperature of several hundred degrees. As a method for dealing with this issue, the art of roughening the untreated copper foil surface by a Zn-containing alloy is disclosed in, for example, Patent Literature (PLT) 1.
Further, as the method of adhering copper foil to an insulating resin, there is proposed a surface-treated copper foil obtained by treating the surface of an untreated copper foil to be adhered to an insulating resin by surface treatment by an electrolytic solution containing at least one type of element selected from among molybdenum, iron, cobalt, nickel, and tungsten and further providing an Ni plating layer or Zn plating layer or (Ni plating layer+Zn plating layer) on this plating layer (see, for example, PLT 2).
The roughened layers containing Zn layers disclosed in PLT's 1 and 2 are effective in the point of improving the adhesion strength between the copper foils and the insulating resins at high temperatures. However, after adhering the copper foils to the insulating resins, when wiring circuits are formed by etching by an acidic solution to obtain circuit boards, even the Zn layers adhering the copper foils and the insulating resins begin to dissolve since zinc is easily dissolved in acid, so the adhesion strength between the copper foil and insulating resin after the circuit formation sharply falls, resulting in the possibility of peeling off of the wiring circuits (copper foils) from the insulating resins during use of the circuit boards. In order to prevent this, dissolution and outflow of the Zn layer is kept to the lowest limit by shortening the etching time. However, an advanced technique and management system are needed for the etching, so the productivity of the circuit boards is lowered and costs are raised.
In this way, in actual circumstances, the roughenings disclosed in PLT's 1 and 2 do not satisfy all of the requirements of adhesion strength with the insulating resins, chemical resistance, and etching properties as described above, and therefore a surface-treated copper foil satisfying these characteristics has not yet been provided.
For this reason, a CCL satisfying all of the adhesion strength, chemical resistance, and etching properties has not yet been provided.
Further, for example, PLT 3 proposes a CCL formed by a surface-treated copper foil and a polyimide film obtained by applying, as the surface treatment of the copper foil, an Ni—Zn alloy plating by using a pyrophosphate bath as the plating bath. It is recognized that, by using the pyrophosphate bath, an Ni—Zn alloy layer excellent in uniformity of film thickness is obtained, and a phenomenon of tin (Sn) submerging into an interface between the circuit and the polyimide base material is hard to occur even when tin plating is carried out for the terminal parts after forming the circuit.
However, in plating using a pyrophosphate bath, as known, phosphorus (P) coprecipitates into the plating layer, so the solubility of the plating layer becomes high due to the coprecipitated P.
When the solubility of the plating layer becomes high, this greatly influences the process of circuit formation by etching. When Sn plating is carried out for the terminal parts in a circuit formed by etching copper foil, the submerging phenomenon of the Sn plating solution (deterioration of chemical resistance) cannot be sufficiently prevented, so the disadvantage arises that the surface-treated layer is deteriorated due to the Sn plating solution, and an adverse influence is exerted upon the adhesion of the wiring circuit.
In recent years, finer pitch of the circuit has been promoted and the width of the wiring circuit has becomes smaller, so the adhesion area between the circuit and the insulating resin is reduced. In such a fine pitch circuit, if submerging of the Sn plating solution occurs, the adhesion of the circuit falls and causes a disadvantage of reliability, therefore a copper foil capable of suppressing this submerging of the Sn plating solution has been demanded.
Here, an example of a process of forming an wiring pattern on a copper clad laminated board in which copper foils are provided on the two surfaces of a thin insulating resin such as a polyimide or the like (hereinafter, sometimes simply referred to as a “laminate board”) by the subtractive method will be simply explained.
First, a photosensitive film (resist) is bonded to one copper foil surface (front surface side) of the laminate board. Using an exposure system which attaches an exposure mask to the photosensitive film surface, the pattern of the exposure mask is transferred (projected) onto the photosensitive film by application of exposure light. Unexposed portions in the photosensitive film are removed by a development process to form a resist pattern (etching resist).
Next, the copper foil in the portions which are not covered by the film resist pattern (exposed) is removed (etched) in an etching process to thereby form wirings on the front surface side. After that, in the etching process, the used film resist pattern is, for example, removed from the surface of the wirings (copper foil) by using an alkali aqueous solution.
Predetermined wirings are formed on the copper foil on the other surface (back surface side) as well by the same process as that described above.
As explained above, after forming wirings on the front and back surfaces, blind via holes for conductively connecting the front surface side wirings (copper foil) and the back surface side wirings (copper foil) are formed.
The blind via holes are formed by a laser beam drilling using CO2 laser or other laser beam on the insulating resin exposed at the front surface side to form holes. In this process of forming holes by a laser, residue (smear) of the insulating resin remains at the bottoms of the holes (back surface side wirings). In order to remove this smear, there is used a potassium permanganate solution or other oxidizing chemicals to remove the smear (perform de-smearing).
Next, in order to conductively connect the front and back copper foils of the resin substrate, in the formed holes, a layer of copper (conductive layer) is formed by electroless plating or electrolytic plating. As the pre-treatment for this, the bottom portions of the holes (back surface side wirings) are treated by a hydrogen peroxide-based soft etchant to remove the surface treated metal of the copper foil. Finally, the bottom portions of the holes (back surface side wirings) subjected to the soft etching and the copper foil formed with holes (front surface side wirings) are conductively connected by electrolytic copper plating to thereby complete the wired substrate.
Note that, it is also possible to perform the process of forming wirings on the back surface side copper foil after forming the blind via holes.